1. Field of the Invention
The present invention relates to a method for fabricating superconductive film on a dielectric substrate by electron beam evaporation. The present invention especially relates to a method for fabricating a superconductive film composed of a compound of bismuth, strontium, calcium, copper and oxygen (Bi.Sr.Ca.Cu.0), or a compound composed of rare earth element, barium, copper and oxygen (RE.Ba.Cu.0).
Recently, considerable effort has been made to use superconductive technology in electronic devices. To achieve such an object, great emphasis is placed on fabricating a superconductive film.
2. Description of the Relevant Art
Since the discovery of a material which shows superconductivity at a relatively high temperature by W. K. Wu et al. ["Superconductivity at 93 K in New Mixed-Phase Y--Ba--Cu--O Compound System at Ambient Pressure", Phys. Rev. Lett., 58, No. 9, (1987)], extensive efforts have been made in various fields of industry to discover new materials which can operate at even higher temperatures, and provide a stable operation. Among many of those new materials, compounds of (RE.sub.1 Ba.sub.2 Cu.sub.3 O.sub.x), where x is 6-7, and compounds of (Bi Sr.Ca.Cu.O) are considered the most promising compounds for electronic applications.
In fabricating a thin film of these compounds, two methods have been mainly tried, one is sputtering and the other is evaporating the materials. Both methods are widely used in electronic device fabrication. Sputtering is used extensively because a variety of compounds can be subjected to glow discharge of gas.
In the sputtering method, a compound of superconductive material is placed in a chamber as a source, and sputtered by an electrical glow discharge generated between the source and a substrate. Such techniques are more completely described in "Preparation of YBa.sub.2 Cu.sub.3 0.sub.7-x Superconducting Thin Films by Rf-Magnetron Sputtering" by T. Aida et al. Japanese Jour. Appl. Phys., 26, No. 9, L1489-L1491 (1987). The ratio of the number of atoms of the elements RE, Ba, Cu in the compound RE.sub.1 Ba.sub.2 Cu.sub.3 0.sub.x,as shown, is 1:2:3. However, if a superconductive material having such an atomic ratio is used for a sputtering source, it has been found that the fabricated film does not show superconductivity, even though the source material exhibits superconductivity. This phenomenon is likely to be attributed to the fact that the sputtering rate of atoms by glow discharge is different for different atoms, and accordingly, the atomic ratio in the sputtered film is shifted from the ideal. To compensate for such a shift in the atomic ratio, it has been proposed to vary the ratio of the components in the source compound as set forth above. This method is discussed in, "Preparation of Y--B--Cu--O Thin Films by Rf-Magnetron Sputtering" by H. Ohkuma et al., Japanese Jour. Appl. Phys., 26, No. 9, L1484--L1486 (1987).
The surfaces of superconductive films formed by sputtering according to the present state-of-the-art technology are found to be rough and lack the smoothness necessary for wiring for electronic devices. This may be due to a back sputtering of the substrate caused by ion bombardment. Further, the sputtering condition varies depending on the structure of the sputtering chamber, arrangement of electrodes in the chamber, pressure of gas, voltage and frequency of the equipment. So, the sputtering condition must be determined for each device used.
With regard to the evaporation method, the source material is evaporated by heating it, and depositing it on a substrate. To heat the material to a temperature of evaporation, a resistance filament heater or an electron beam is used. Recently, electron beam evaporation has been widely used. The surface of the film fabricated by an evaporation method is smooth. Therefore, the evaporated film is preferable for use in electronic devices.
Generally, to fabricate a film of a compound by an evaporation method, component materials, usually metals, are put in respective crucibles, and evaporated separately from each other but usually at the same time toward the substrate. The elements are reacted with each other and form the desired compound on the substrate or on the way to the substrate. By controlling the temperature of each crucible the evaporation rate of each element can be varied. Accordingly, the deviation of the atomic ratio from the desired value can be avoided, but generally, annealing is necessary to obtain a complete compound. Without annealing the produced film does not exhibit superconductivity. Or in order to exhibit superconductivity it is necessary to lower the temperature of the composition to that of liquid helium. The above situation may be seen, for example in "As-deposited Superconducting Y--Ba--Cu--O Thin Films on Si, Al.sub.2 O.sub.3, and SrTi0.sub.3 Substrates" by R. M. Silver et al., Appl. Phvs. Lett., 52, (25), (1988).
From a view point of applying superconductivity technology to electronic devices, the operating temperatures of the superconductive film is desirable to be as high as possible. At least it is desirable to operate at the temperature of liquid nitrogen, and it is preferable not to use annealing, because many electronic devices, such a semiconductor devices, do not lend themselves to being heated to high temperatures.